Flat rooflight sizes: standard sizes and how to choose

Flat rooflights are sold in standard sizes so the kerb can be built before the glass arrives. Choosing from standard sizes is usually quicker and cheaper than a one-off size, and it lets you plan the roof around a known opening.

Our standard sizes

All sizes are in millimetres, width x length.

Width	Lengths available
600	600, 900, 1200, 1500
800	800, 1000, 1200, 1800
900	900, 1200
1000	1000, 1200, 1500, 2000, 2500, 3000
1200	1200
1500	1500

Before the kerb is built, send us the planned inside and outside kerb dimensions and we will confirm which size fits. If the kerb already exists, measure it as shown in our measuring guide.

How to choose a size

Start from the joists. Roof joists are often set at 400 or 600 mm centres. A rooflight width that fits between joists, or needs only one joist trimmed, keeps the structural work simple. Ask your builder before you fix the size.
Think about the room. One larger rooflight gives a strong pool of light; two or three smaller ones spread it more evenly along a long kitchen or extension.
Think about the view up. Long narrow rooflights, such as 1000 x 2500 or 1000 x 3000 mm, work well over a kitchen island or along the back wall of an extension.
Check weight and handling. Larger triple glazed units are heavy and need more than one person, or lifting equipment, to place safely. Plan access to the roof.

When a standard size does not fit

If your kerb is already built and does not match a standard size, measure it carefully and send us the inside and outside dimensions. We will tell you whether a standard size can be used with a small change to the kerb, or price a made to measure unit.

Sizes by room

Room	Sizes people often choose	Why
Bathroom, landing or utility room	600 x 600, 600 x 900, 800 x 800 mm	Daylight without a large opening; usually fits between joists
Kitchen island	1000 x 2000, 1000 x 2500, 1000 x 3000 mm	A long strip of light along the worktop
Dining or living area	1000 x 1000, 1200 x 1200, 1500 x 1500 mm	A square pool of light over a table or seating
Long rear extension	Two or three matching sizes in a row	Spreads light evenly from front to back
Hallway or corridor	600 x 1200, 600 x 1500 mm	A narrow shape follows the space

These are common starting points, not rules. Your joist layout, the roof size and the look you want matter more. Our guide to skylights for kitchens goes further on placement.

Standard or made to measure

	Standard size	Made to measure
Price	Fixed price per size	Priced to your size
Planning	Build the kerb to a known size	Measure the finished kerb first
Best for	New roofs and extensions	Existing kerbs and replacements

Light from a smaller rooflight

The opening in the ceiling does not have to be straight-sided. Splaying the plastered reveal, so it gets wider towards the room, spreads the light from a rooflight further across the ceiling and makes a modest size feel larger. A light-coloured reveal helps too. Ask your builder about this while the roof is being framed, because it is hard to change later.

Heavier than it looks

Glass weighs about 2.5 kg per square metre for each millimetre of thickness, and a triple glazed rooflight has three panes. A large size can need lifting equipment or several people to place safely on a roof. Plan how it will get from the delivery vehicle to the kerb before you choose the largest size.

Questions we are asked

Is the size the glass or the opening?

The size name is a reference, not a kerb dimension. Send us your kerb sizes and we will confirm the fit before anything is made.

Can I use two rooflights instead of one large one?

Yes. Two or three smaller rooflights spread light more evenly along a long room and can be easier to frame between joists.

What is the most popular rooflight size?

It depends on the room. Small square sizes are common for bathrooms and landings, and long sizes such as 1000 x 2000 mm are popular over kitchen islands.